1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, the present invention relates to a structure of a cathode/anode lead plate attached to a protection circuit board for a secondary battery.
2. Description of the Related Art
Recently, compact and lightweight portable electronic devices, such as cellular phones, portable computers and camcorders, etc., have been actively developed and produced. The portable electronic devices include a built-in battery pack so as to be operated at locations where power is not available. A secondary battery that can be charged and discharged is used as the battery pack for economical reasons. Typical secondary batteries include nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni—MH) batteries, lithium (Li) batteries and lithium ion (Li-ion) secondary batteries, etc. Especially, lithium ion secondary batteries have operating voltages that are three times that of nickel-cadmium (Ni—Cd) batteries or nickel-hydrogen (Ni—MH) batteries that are usually used to supply power to portable electronic devices. In addition, lithium ion secondary batteries have been widely used because they have a higher energy density per unit weight as compared with other types of batteries.
Lithium ion secondary batteries use lithium group oxides as active cathode materials, and carbon materials as active anode materials. Generally, lithium secondary batteries are divided into liquid electrolyte batteries and polymer electrolyte batteries, depending on their electrolyte. A lithium ion battery uses a liquid electrolyte, and a lithium polymer battery uses a polymer electrolyte. Lithium secondary batteries have been fabricated in various shapes, such cylinder shaped, angled shaped and pouch shaped, etc.
Generally, in a fabrication process of a secondary battery, an electrode assembly is formed by laminating a cathode plate where the cathode active material is coated on a cathode collector and a cathode tap is connected, an anode plate where the anode active material is coated on an anode collector and an anode tap is connected, and a separator and winding them. The electrode assembly is put into a can type case for the secondary battery. Then, a bare cell is fabricated by injecting the electrolyte and sealing the can type case with a cap assembly. The bare cell is electrically coupled to a protection circuit board, and the bare cell and the protection circuit board are put into a mold and melded by melted resin, and thus, a resin molding type secondary battery is completed.
The protection circuit board and the bare cell are electrically connected by a lead plate. The protection circuit board is formed with a conducting pad which is connected to the lead plate. The conducting pad and the lead plate are soldered together.
But, when connecting the conducting pad to the lead plate, the solder is melted so that the solder flows, and the solder may not solidify so as to be flat. Moreover, due to the shape of the conducting pad and the lead plate, a spread of the solder may be insufficient. In this case, the protection circuit board is connected to the bare cell at an angle, resulting in many problems.
For example, when the protection circuit and bare cell are connected at an angle, a next molding process results in a terminal of the protection circuit board being at an angle. If the terminal of the protection circuit board is inclined to one side, a space is formed by the inclined protection circuit board, allowing melted resin to penetrate into the space, resulting in a flash being formed. If the flash covers a surface of the terminal, the terminal can not be electrically connected to the electric device.
Moreover, a lead plate connected to the conducting pad often has a rectangular shape, so that a combined force of the conducting pad and lead plate is weak. Accordingly, the lead plate and conducting pad may be separated from each other due to the weak combined force.